1. Field of the Invention
The present invention relates to a display device comprising current control light emitting elements such as organic EL (Electro-Luminescence) elements laid out in the form of a matrix, wherein luminance of the elements is controlled in accordance with currents applied thereto.
The present invention also relates to a current control driver for driving current control elements in a device, wherein the elements are laid out in the form of a matrix.
2. Description of the Related Art
In an active matrix image display device, a plurality of pixels are laid out in the form of a matrix and an image is displayed by controlling intensity of light for each of the pixels according to luminance information supplied thereto. Liquid crystal display devices and organic EL display devices are known as specific examples of image display devices of this type. Liquid crystals are used as display elements that constitute each pixel in a liquid crystal display device, while organic EL elements are used in an organic EL display device. Organic EL elements that constitute each pixel of an organic EL display device are so-called self-luminous elements, and organic EL display devices are more advantageous than liquid crystal display devices with regard to higher image visibility, non-necessity of backlight, and higher response speed. The luminance of each light emitting element in an organic EL display device is controlled by an amount of current.
In an active matrix method, an active element inside each pixel, which is generally a TFT (Thin Film Transistor) as a type of FET (Field Effect Transistor), controls the amount of current flowing through a light emitting element of the pixel. U.S. Pat. No. 5,684,365 describes an example of an active matrix organic EL display device, and FIG. 31 shows an equivalent circuit thereof for one pixel (hereinafter referred to as Conventional Example 1). Each pixel in this circuit comprises an organic EL element OLED as a light emitting element, a first thin film transistor TFT1, a second thin film transistor TFT2, and a capacitor C. Since organic EL elements provide current rectification in many cases, organic EL elements are called organic light emitting diodes (OLEDs). Therefore, a symbol for a diode is used for the light emitting element OLED in FIG. 31. However, the light emitting element in this case is not necessarily this OLED, and any element whose luminance is controlled by the amount of current flowing therein may be used. In addition, the light emitting element does not necessarily carry out current rectification. In the example shown in FIG. 31, the source of P-channel TFT2 is connected to VDD (a power supply voltage), and the cathode of the light emitting element OLED is connected to a ground voltage. The anode of OLED is connected to the drain of TFT2. The gate of N-channel TFT1 is connected to a scanning line Scan while the source thereof is connected to a data line Data. The drain of TFT1 is connected to the capacitor C and the gate of TFT2.
In order to cause the pixel to operate in the above configuration, the scanning line Scan is firstly changed to a selected state and a voltage Vdata representing luminance information is applied to the data line Data. The transistor TFT1 then becomes conductive, and the capacitor C is either charged or discharged, to match a gate voltage of TFT2 with the data voltage Vdata. When the scanning line Scan is changed to a non-selected state, the transistor TFT1 becomes OFF. The transistor TFT2 is electrically disconnected from the data line Data, but the gate voltage of TFT2 is stably maintained by the capacitor C. A current flowing through OLED via TFT2 takes a value corresponding to a gate-source voltage Vgs of TFT2, and the light emitting element OLED continues to emit light at luminance according to the amount of current supplied via the transistor TFT2.
If Ids denotes a current flowing between the source and the drain of TFT2, Ids is the drive current flowing through OLED. If TFT2 operates in a saturation region, Ids is expressed by the following equation:
                              I          ds                =                                            1              2                        ⁢            μ            ⁢                                                  ⁢                          C              ox                        ⁢                          W              L                        ⁢                                          (                                                      V                    gs                                    -                                      V                    th                                                  )                            2                                =                                    1              2                        ⁢            μ            ⁢                                                  ⁢                          C              ox                        ⁢                          W              L                        ⁢                                          (                                                      V                    data                                    -                                      V                    th                                                  )                            2                                                          (        1        )            where Cox is a gate capacitance per unit area and represented by the following equation:
                              C          ox                =                                            ɛ              0                        ⁢                          ɛ              r                                d                                    (        2        )            In the above equations, Vth, μ, W, L, ∈0, ∈r, and d respectively refer to a threshold value of TFT2, the mobility of carriers, a channel width, a channel length, a vacuum dielectric constant, the relative permittivity of a gate insulator, and the thickness of a gate insulator.
According to Equation (1) above, Ids is controlled by the voltage Vgs written into each pixel, and the luminance of the light emitting element OLED is controlled as a result. The reason why TFT2 is made to operate in the saturation region is that Ids is controlled only by Vgs in the saturation region and is not dependent on a drain-source voltage Vds. Therefore, the drive current Ids of a predetermined amount can be supplied to OLED even in the case where Vds fluctuates due to variations in the characteristics of OLED.
As has been described above, OLED in the circuit configuration shown in FIG. 31 continues to emit light at constant luminance once Vgs has been supplied thereto during one scanning period (one frame) until Vgs is supplied next time. By arranging a plurality of pixels 3 of such a type in the form of a matrix as shown in FIG. 32, an active matrix display device can be configured. As shown in FIG. 32, scanning lines Scan1 to ScanN for pixel selection during a predetermined scanning period (such as a frame period according to the NTSC standard) and data lines Data that feed luminance information (data voltage Vdata) for driving the pixels are laid out in the form of a matrix in a conventional display device. The scanning lines Scan1 to ScanN are connected to a scanning line driver 1 while the data lines Data are connected to a data line driver 2. A desired image can be displayed by repeating application of Vgs from the data lines Data by use of the data line driver 2 while sequentially selecting the scanning lines Scan1 to ScanN by use of the scanning line driver 1.
In a simple matrix display device, a light emitting element of each pixel emits light only at the instant that it is selected while the light emitting element of each pixel in the active matrix display device shown in FIG. 31 continues to emit light after writing has been completed. Therefore, instantaneous luminance can be lower than in a simple matrix display device and the amount of current for driving each light emitting element can be smaller, which are advantageous especially for large high-definition display devices.
As has been described above, TFTs that can be easily formed on glass substrates are generally used as active elements in an active matrix organic EL display device. Amorphous silicon and polysilicon used to form TFTs are not as crystalline as single crystal silicon, and the electric current conduction mechanism thereof is difficult to control. Therefore, TFTs formed by amorphous silicon or polysilicon show larger characteristic variations. Especially, in the case where polysilicon TFTs are formed on a comparatively large glass substrate, a laser annealing method is generally adopted in order to avoid problems such as deformation of the glass substrate caused by heat. However, uniform irradiation of laser energy on glass substrates of such a size is difficult, and variations in crystallization of polysilicon cannot be prevented from occurring, depending on positions in the substrate.
As a result, the threshold value (Vth) varies from pixel to pixel even among TFTs formed on the same substrate, and variation exceeding 1V is not rare in some cases. In such a case, Vth varies among pixels even if the signal voltage Vdata applied thereto is the same. Therefore, as shown by Equation (1) above, the current Ids flowing through OLED varies greatly from pixel to pixel, becoming far from a desired value. Consequently, high image quality, which is expected from a display device, cannot be achieved. The same phenomenon is observed not only in the voltage Vth but also in variations in carrier mobility μ. In addition, variations in each of the parameters cannot be avoided not only among pixels but also among production lots and products. In such a case, the data line voltage Vdata corresponding to the desired current Ids to flow through OLED needs to be set for each product according to resultant variations of the respective parameters in Equation (1). However, this process is unrealistic in mass production processes of display devices, and changes in the TFT characteristics caused by operating temperature, as well as temporal changes in the TFT characteristics caused by long term use, are extremely difficult to deal with.
U.S. Pat. No. 6,501,466 describes a configuration combining a power source and a current mirror circuit (hereinafter referred to as Conventional Example 2), in order to solve the problems of Conventional Example 1. The configuration is shown in FIG. 33. In Conventional Example 2, a current Iw corresponding to luminance is supplied between the source and the drain of TFT1 via TFT3, and TFT4 is in a conductive state at this time. A gate-source voltage of TFT1 becomes a value corresponding to the current Iw, and a capacitor C is set to the voltage. Thereafter, TFT4 becomes nonconductive and the voltage of the capacitor C, that is, a gate-source voltage of TFT2 is retained. Therefore, a current in accordance with the gate-source voltage flows between the source and the drain of TFT2 and to an organic EL element.
In the circuit of Conventional Example 2, the write current Iw to be applied from a data line needs to be set large in many cases compared to a current Idrv to flow through the light emitting element OLED. This is because the current to flow to OLED is generally up to several μA or the like at a maximum luminance but the current is approximately a dozen nA or slightly more for tones close to a minimal value of a 256-tone display, for example. Therefore, it is generally difficult for such a small current to be supplied accurately to the pixel circuit via the data line having a large capacitance.
In order to solve this problem, the current Iw can be increased by setting a value of (W2/W1)/(L2/L1) to be small where W1, W2, L1, and L2 respectively denote channel widths of TFT1 and TFT2, and channel lengths of TFT1 and TFT2 in the circuit shown in FIG. 33. However, in order to cause the large current Iw to flow, the value of W1/L1 needs to be increased for TFT1. In this case, the channel width W1 needs to be increased inevitably, since various limitations apply in decreasing the channel length L1. As a result, TFT1 occupies a large portion of a pixel area.
This fact usually means that the area that emits light becomes smaller in an organic EL display device in the case where pixels are uniform in size. As a result, reliability is lowered due to increase in current density, and power consumption increases due to increase in drive voltage. In addition, graininess is worsened due to the decrease in the light emitting area. Furthermore, this fact leads to reduction in the pixel size, which prevents the display from having higher resolution.
In order to solve these problems, U.S. Patent Application Publication No. 2003/0107560 proposes a driver (herein after referred to as Conventional Example 3) wherein a TFT is shared between pixels and a large size TFT is used to allow a large current to flow while a TFT area per pixel can be reduced. Hereinafter, the driver of Conventional Example 3 will be described with reference to FIG. 34. For the sake of simplification, circuits for two neighboring pixels in a column (pixels 1 and 2) are shown in FIG. 34.
In FIG. 34, a circuit P1 for the pixel 1 has an OLED (organic EL element) 11-1, a TFT 12-1, a capacitor 13-1, a TFT 14-1, and a TFT 15-1. The anode of OLED11-1 is connected to a positive power supply VDD. The drain of TFT12-1 is connected to the cathode of OLED11-1 while the source thereof is grounded. The capacitor 13-1 is connected between the gate of TFT12-1 and the ground (reference voltage point). The drain of TFT14-1 is connected to a data line 17 while the gate thereof is connected to a first scanning line 18A-1. The drain of TFT15-1 is connected to the source of TFT14-1 while the source thereof is connected to the gate of TFT12-1. The gate of TFT15-1 is connected to a second scanning line 18B-1.
Likewise, a circuit P2 of the pixel 2 has an OLED 11-2, a TFT 12-2, a capacitor 13-2, a TFT 14-2, and a TFT 15-2. The anode of OLED11-2 is connected to a positive power supply VDD. The drain of TFT12-2 is connected to the cathode of OLED11-2 while the source thereof is grounded. The capacitor 13-2 is connected between the gate of TFT12-2 and the ground. The drain of TFT14-2 is connected to the data line 17 while the gate thereof is connected to a first scanning line 18A-2. The drain of TFT15-2 is connected to the source of TFT14-2 while the source thereof is connected to the gate of TFT12-2. The gate of TFT15-2 is connected to a second scanning line 18B-2.
A so-called diode connection transistor TFT 16, whose gate and drain are electrically short-circuited, is shared between the circuits P1 and P2 for the two pixels. In other words, the drain and the gate of TFT16 are connected to the source of TFT14-1 and to the drain of TFT15-1 in the circuit P1 while the drain and the gate of TFT16 are also connected to the source of TFT14-2 and the drain of TFT15-2 in the circuit P2. The source of TFT16 is grounded.
In the example shown in FIG. 34, N-channel MOS transistors are used for TFT12-1 and TFT12-2 as well as for TFT16 while P-channel MOS transistors are used for TFT14-1 and TFT14-2 as well as TFT15-1 and TFT15-2.
In the pixel circuits P1 and P2 having the above configuration, TFTs 14-1 and 14-2 function as first scanning switches for selectively supplying a current Iw from the data line 17 to TFT16 while TFT16 functions as a converting unit that converts the current Iw applied from the data line 17 via TFT14-1 or TFT14-2 into a voltage. At the same time, TFT16 forms a current mirror circuit together with TFTs 12-1 and 12-2 that will be described later. The transistor TFT16 can be shared between the circuits P1 and P2 because TFT16 is used only at the time of applying the write current Iw.
The transistors TFT15-1 and TFT15-2 function as second scanning switches for selectively supplying the voltage converted by TFT16 to the capacitor 13-1 or 13-2. The capacitors 13-1 and 13-2 function as retaining units that retain the voltage converted from the current by TFT16 and supplied via TFT15-1 or TFT15-2. The transistors TFT12-1 and TFT12-2 function as driving units that cause OLEDs 11-1 or 11-2 to emit light by converting the voltage retained by the capacitor 13-1 or 13-2 into a current and by supplying the current to OLEDs 11-1 or 11-2. The elements OLED11-1 and OLED11-2 are electric optical elements whose luminance changes according to the current flowing therethrough.
The operation of luminance data writing in the driver having the above configuration will be described next. How luminance data are written in the pixel 1 will be described first. A current Iw in accordance with the luminance data is supplied to the data line 17 while the scanning lines 18A-1 and 18B-1 are both selected (that is, scanning signals ScanA1 and ScanB1 are both at a low level in this case). The current Iw is supplied to TFT16 via TFT14-1 that is in a conductive state. A voltage corresponding to the current Iw occurs at the gate of TFT16, by the current Iw flowing through TFT16. The voltage is retained by the capacitor 13-1.
The current corresponding to the voltage retained by the capacitor 13-1 is supplied to OLED11-1 via TFT12-1. In response, OLED11-1 starts to emit light. When the scanning lines 18A-1 and 18B-1 are set to non-selected states (that is, the scanning signals ScanA1 and ScanB1 are both at a high level), the writing operation of the luminance data to the pixel 1 is completed. The scanning line 18B-2 is in a non-selected state during this operation. Therefore, OLED11-2 of the pixel 2 is emitting light according to a voltage retained by the capacitor 13-2, and the state of light emission from OLED11-2 is not affected by the writing operation to the pixel 1.
The operation of luminance data writing to the pixel 2 will be described next. A current Iw corresponding to luminance data is supplied to the data line 17 while the scanning lines 18A-2 and 18B-2 are both in selected states (that is, scanning signals Scan A2 and B2 are at low level). A voltage corresponding to the current Iw is generated at the gate of TFT16 by the current Iw flowing through TFT16 via TFT14-2. The voltage is retained by the capacitor 13-2.
A current corresponding to the voltage retained by the capacitor 13-2 is supplied to OLED11-2 via TFT12-2. In response, OLED11-2 starts to emit light. The scanning line 18B-1 is in a non-selected state during this operation. Therefore, OLED11-1 of the pixel 1 is emitting light according to the voltage retained by the capacitor 13-1, and the state of light emission from OLED11-1 is not affected by the writing operation to the pixel 2.
As has been described above, TFT16 that carries out current-voltage conversion is shared between the two pixels in the driver of Conventional Example 3. Therefore, one transistor can be omitted for every two pixels. The current Iw flowing through the data line 17 is extremely large compared to currents flowing through OLEDs (organic EL elements), and the current-voltage converting TFT16 that directly deals with the large current Iw has a large size which occupies a large area. However, the current-voltage converting TFT16 is shared by the two pixels in this example, which enables TFT area reduction.
However, the combinations of TFTs which are shared between pixels are fixed in the driver in Conventional Example 3 described in U.S. Patent Application Publication No. 2003/0107560. Therefore, uneven display caused by differences in FET characteristics among the pixels cannot be avoided.
Although examples that use TFTs as active elements for controlling currents flowing through light emitting elements have been described above, the same problem occurs even if other active elements are used. In addition, the same problem also occurs not only in display devices but also optical scanning reading apparatuses or optical scanning recording apparatuses that adopt light emitting elements laid out in the form of a matrix and generate reading light or recording light of constant luminance whose value can be changed through sequential scanning of the elements.